A line-locked video system is a digital system that acquires a line-by-line analog signal. An example of such a system would be a video decoder. Each line signal acquired by the system contains a synchronizing pulse and a “color burst,” neither of which carry information that is displayed on the screen. The synchronizing pulse is utilized so that a genlock PLL can locate the analog signal and lock onto it. The “color burst” is a series of eight cycles at the subcarrier frequency which appear in blanking intervals for synchronizing the television receiver to the chrominance signal. The display portion of the signal is transferred after both the synchronizing pulse and the “color burst” portion of the signal. Due to the continuous changing of the clock(CLK) signal from the genlocking step, disturbance is usually created in the chrominance subcarrier PLL which will result in a phase shift and consequently in color stripe artifact. A more detailed background description is set forth below.
The composite video signal contains information which is used by a video system to generate a video picture on a display, monitor or television. Each period, within the horizontal portion of a composite video signal contains information representing one horizontal output line which is to be output on the video display, monitor or television. Each horizontal period includes a horizontal synchronization pulse, a burst signal and a video information signal. In many video transmission systems, color or chrominance information is represented by a particular phase of the chrominance subcarrier signal that is amplitude modulated with color information. The horizontal synchronization pulse is used by a phase-locked loop to synchronize the system for displaying the next horizontal line of video information. The burst signal is used to synchronize the phase and frequency of a local oscillator to that of the encoding oscillator so that color information can be “decoded” into its baseband color differential components.
A video picture or frame is made up of a number of horizontal lines included within the video display. To display a video picture or frame the video system begins at the top of the screen and displays the information within the composite video signal one horizontal line at a time. The information for each horizontal line is contained within a horizontal period of the composite video signal. After each horizontal period, the video system moves to the next line and displays the information within the next horizontal period of the composite video system. This continues until the video system reaches the bottom line on the video display. After displaying the video information on the bottom line of the video display, the video system must reset itself to the top of the display in order to begin displaying the next frame. In order to allow the system to reset itself to the top of the video display, a vertical blanking period is included within the composite video signal after the video information for each frame. This vertical blanking period allows the video system to reset to the top of the video display and begin displaying the information for the horizontal lines of the next frame. Therefore, a number of horizontal periods, enough to comprise a frame or screen, are strung together, within the composite video signal. Between each frame, the composite video signal includes a vertical blanking period which allows the video system to perform a vertical reset and prepare to display the next frame by moving back up to the top of the video display.
In this type of system, a genlock PLL device is used to lock and align with the synchronizing pulse portion of the signal. In the receiver portion of this system, a chrominance subcarrier PLL is utilized to detect and lock onto the “color burst” portion of the analog signal. These two PLLs share a common CLK signal. When the genlock PLL locks and aligns the video signal, the CLK is adjusted accordingly. Because of this arrangement, the problem which is readily encountered is a phase shift disturbance of the SINE wave in the chrominance subcarrier PLL in the receiver. This disturbance is caused by the simultaneous locking of the genlock device, thereby causing continuous changing of the CLK signal.